1. Field
This document relates to a phase correction method, and more particularly, to a non-linear phase correction method.
2. Related Art
Magnetic resonance imaging (MRI) is used to acquire an accurate image of interior parts of a human body. MRI is based on a field of physics in nuclear magnetic resonance (NMR). Nuclei including an odd number of neutrons or an odd number of protons usually have a net magnetic moment, and are NMR active.
Echo planar imaging (EPI), which is a high-speed MRI technique, is widely used in various fields including functional MRI (fMRI), diffusion tensor imaging (DTI), which is used to find out the position and direction of white matter fibrous tissues, cardiac function measuring imaging, diffusion-weighted imaging (DWI), and perfusion-weighted imaging.
EPI is generally classified into two types, a spin echo and gradient echo, depending on a method of obtaining an echo signal.
Typically, gradient echo EPI is used to acquire images related to fMRI and cardiac functions, while spin echo EPI is used to acquire images related to DTI, DWI, and PWI.
EPI needs to be used up to a critical value of hardware because of the fast signal acquisition. Also, since a signal is acquired by a shingle shot, EPI is sensitive to uniformity in a main magnetic field, variation in magnetic susceptibility, and chemical transition. Due to the above limitation and characteristics, various artifacts such as Nyquist ghost and geometric distortion may be produced in finally reconstructed images.
A correction method of removing artifacts inherent in high-speed EPI is commonly classified into a linear phase correction method and a non-linear phase correction method.
For a linear phase correction method, there is one widely applied approach that a phase is first found using a navigator echo signal, which is additionally acquired to use reference information suggested in an article by Heid O., entitled “Robust EPI Phase Correction,” In Proceedings of the Society of Magnetic Resonance in Medicine, p. 2014, 1997, and an acquired signal is corrected based on a phase found from the first found phase. Another widely applied approach for the linear phase correction method includes reconstructing an image by drawing even and odd numbers of signals individually as suggested in an article by Buonocore M. H. and Gao L., entitled “Ghost Artifact Reduction for Echo Planar Imaging Using Image Phase Correction,” Magn. Reson. Med., 38, pp. 89-100, 1997, generating two-dimensional phase images, selecting one row of the images, and applying the selected row to all of the signals for the correction.
For a non-linear phase correction method, there is one approach that sampled points of signals acquired by EPI, which applies a phase gradient magnetic field, are individually corrected using a phase of an additional reference information signal acquired by EPI, which is removed of a phase gradient magnetic field. Such a non-linear phase correction method contains more information than the linear phase correction method, and thus, the non-linear phase correction method is considered effective to remove artifacts produced in high-speed EPI.
However, when the non-linear phase correction method is applied, in gradient echo EPI, numerous echo signals acquired from a certain sectional plane often have different levels of Dephasing due to variations in uniformity of a main magnetic field and magnetic susceptibility. The Dephasing of the echo signals may generate a streak artifact.